Ultrasound imaging involves the display of information obtained from reflections of (echoes of) pulses of ultrasonic waves directed into the body. These echoes contain information about the underlying structure of the tissue and blood flow in vessels in the region exposed to ultrasound waves. One use of the ultrasound images is for contrast agent imaging for a heart study.
FIG. 1 illustrates the steps associated with a conventional contrast agent imaging technique for a heart study. First, the tissue 10 is insonated (i.e., exposed to ultrasound waves) via an ultrasound transducer probe 12 in the absence of a contrast agent, and a single image frame is acquired and stored in a first location within an image frame memory 14. The pre-injection image is acquired at a known point in the heart cycle by using a QRS trigger, such as described in U.S. Pat. No. 5,255,683 (Monaghan), incorporated in its entirety herein by reference. Next, the contrast agent is injected into the patient and another single image frame is acquired after the contrast agent has perfused into the heart vessels. This post-injection image is stored in a second location in the image frame memory 14.
Referring to FIG. 2, the two images are then processed by an offline computer 16. The computer aligns the two images as best as possible and performs a subtraction, thereby obtaining a "contrast-only difference image" (not shown). The contrast-only difference image is used for rendering a diagnosis.
To further explain the meaning of a contrast-only difference image, the subtraction of two ultrasound image frames taken at different points in time, but at the same times in the physiological cycle is referred to generically as a "difference image. " A difference image is typically the absolute value of the image differences. Difference images obtained in contrast agent imaging are slightly different than generic difference images. When obtaining a contrast-only difference image of two images which are presumed to be perfectly aligned, pixel brightness differences which are less than zero are typically zeroed instead of being assigned an absolute value, since it is presumed that the post-injection image should always be brighter than the pre-injection image. That is, any pixels in the pre-contrast image which are brighter than the corresponding pixels in the post-contrast image are presumed to represent noise.
The pre-injection and post-injection images may be further improved via filtering. Filtering is useful in reducing acoustic "speckle." Acoustic speckle is caused by interference patterns from wavefronts. The interference patterns cause constructive or destructive interference. Acoustic speckle shows up as bright spots and black holes on the image. In one conventional speckle reduction technique, a plurality of pre-contrast and post-contrast images are obtained at the same point in successive cardiac cycles. Again, the QRS trigger is used as a reference point for identifying when the images should be captured. The plurality of pre-contrast and post-contrast images are then separately filtered using a conventional filtering technique, such as averaging a plurality of such images. The result is a speckle-reduced pre-contrast image and a speckle-reduced post-contrast image obtained at a single point in the cardiac cycle. The resultant filtered images are then subtracted from each other using the offline computer 16.
There are significant deficiencies in the conventional contrast agent technique. For example, there is a significant time period between acquisition of the pre-injection and post-injection image frames. In this time period, the operator may have moved the probe 12, or the patient may have moved, so that the probe 12 is not in the same exact location during the acquisition of the two images. Furthermore, it is not possible to determine that relative movement occurred during the image acquisition. The offline computer 16 can move the pre-injection image up or down, or left or right, to match up with the post-injection image. Thus, while some movement can be corrected, it is frequently impossible to exactly align the images. One reason why the images often cannot be perfectly aligned is that the images are 2-D representations of 3-D structures so that mere X-Y axis movements will not necessarily cause the images be aligned.
FIGS. 3A-3C show one conventional alignment scheme which uses a region-of-interest approach to automatically align the images. While the region of interest may be alignable, this does not mean that the rest of the image is necessarily aligned. See FIG. 3C which shows a perfectly overlapping region of interest, but poor overlap in other image regions. If the images cannot be accurately aligned, the resultant subtraction image will be inaccurate.
Retesting is sometimes necessary if alignment problems occur during offline processing which cannot be corrected by the computer 16. Retesting is expensive and often impractical. When the patient is suffering from an acute condition, the time lost in the process may cause a significant problem in patient management. Furthermore, the offline alignment process may introduce errors into the heart study, particularly if the operator makes a mistake in aligning the images.
At least three significant problems thus exist with the conventional technique, namely that the results are not available instantly, the patient must be retested if the images cannot be sufficiently aligned, and the offline alignment process may introduce additional errors in the heart study. Accordingly, there is a significant and unmet need for an ultrasound imaging system for contrast agent imaging which provides instant contrast images, minimizes the need for retesting, and reduces the opportunity for operator error. Furthermore, there is also a need for a difference image processing scheme which achieves such needs. The present invention fulfills all of these needs.